In Tim MCcreights book 'complete metalsmith' it states in the
appendix/alloys section that 18k (75au25pd) white gold has a
melting temperature of 904 deg c. What I don't get is that gold and
palladium have melting points higher than that. So how can the
alloy have that melting point?

There is a fancy name for the lower melting temperature of the alloy
vs. its ingredients. “Eutectic”. This is common, sterling silver is
the same way. You can add yellow 18k to split the difference color
wise. What it will really act like in another matter that will vary
depending on the exact alloys involved.